This invention relates to the field of electrical capacitors and more particularly to electrically variable piezoelectric capacitors manufacturable on thick or thin film hybrid microelectronic circuit substrates.
Hybrid microelectronic circuits are typically manufactured using either a thick film or a thin film process. In this thick film process, conductors, resistors, dielectrics, and insulators are silk screened onto a thin ceramic substrate, dried, and fired. Solder paste is then screened on at appropriate locations and discrete parts are attached to the substrate by heating the substrate and reflowing the solder. In the thin film process conductors are formed by applying a conductive material to the substrate surface and then etching the surface at predetermined locations using a photolithgraphic process. When a particular circuit, such as a voltage controlled oscillator, requires an electrically variable capacitor, a varactor diode is normally used because of its small size and ease of attachment to a hybrid microelectronic circuit substrate.
Varactor diodes have disadvantages, however, the most significant is their nonlinear voltage-capacitance response. It is a well known principle that nonlinear devices generate harmonic frequencies. In radio circuits, where the spectral purity of the transmitted signal is extremely important, any nonlinear device can generate unwanted harmonics which can be unintentionallly amplified and transmitted. Thus, when varactor diodes are used in radio circuits, extreme care must be taken to insure that any harmonic frequencies that they generate are not radiated.
Another disadvantage of varactor diodes and other nonlinear capacitors is that they are two terminal devices. Therefore, the capacitance control voltage and the signal voltage cannot be separate and isolated voltages, but must be applied across the same two terminals. Therefore, the signal voltage must be combined with the control voltage.
Electrically variable capacitors have also been manufactured from piezoelectric elements. A typical prior art piezoelectric variable capacitor includes two bimorph piezoelectric elements. Each element includes two piezoelectric layers of different polarizations bonded together with a conductive control plate in between the layers. One surface of each element has a conductive control plate while the other surface has a centrally located capacitor plate with a lead extending to the edge of the element. Each capacitor plate is surrounded by a narrow insulating area which in turn is surrounded by another control plate. The two elements are then bonded together at their capacitor plate surfaces. A control voltage, appropriately applied to the various control plates, causes the elements to flex at their centers, thereby moving the two capacitor plates further apart and varying the capacitance accordingly.
Although electrically variable piezoelectric capacitors are more linear than varactor diodes, the prior art piezoelectric capacitor is not well suited for either attachment to a hybrid microelectronic circuit substrate or for manufacturing using existing hybrid microelectronic circuit manufacturing technology.